A stabilizer used for a vehicle such as an automobile is a device for ensuring the roll stiffness of the vehicle. FIG. 1 is a perspective view showing the structure of a stabilizer connected to the suspensions of a vehicle. A stabilizer 10 includes, for example, a torsion part 11, arm parts 12, and shoulder parts 13, and is substantially in a U-shape. The stabilizer 10 is arranged such that the torsion part 11 is fitted to a vehicle body (not shown) through bushes 3, and the tip end portions of the arm parts 12 are connected to suspensions 1 through stabilizer links 2. Incidentally, tires are fitted to the left and right axle portions 1A of the suspension 1.
In a process of manufacturing a stabilizer, heat treatment, such as quenching and tempering, is performed on a half-finished stabilizer having a torsion part, arm parts, and shoulder parts. Tempering has conventionally been performed in a furnace by heating a half-finished stabilizer. To take the place of such a tempering process, a tempering process by performing current application heating of a half-finished stabilizer is presented. To perform current application heating of a half-finished stabilizer, the entire half-finished stabilizer is heated by providing electrodes at the end portions of the left and right arm parts and applying current between electrodes.
However, while performing current application heating of a half-finished stabilizer, current tends to flow along the shortest route of the half-finished stabilizer. If the half-finished stabilizer has a curved part, such as a shoulder part, as the inner portion of the curved part forms the shortest route shorter than the outer portion of the curved part, current tends to flow not in the outer portion of the curved part but in the inner portion of the curved part. Accordingly, the inner portion of the curved part is heated more than the outer portion of the curved part and the temperature becomes higher at the inner portion of the curved part than at the outer portion of the curved part during tempering. Thus, a significant difference in temperature occurs between the inner portion and the outer portion of the curved part in the tempering, and the hardness of the half-finished stabilizer after the tempering becomes significantly different between the inner portion and the outer portion of the curved part. As a result, the irregularity of hardness in the half-finished stabilizer becomes significant, and a difference in the mechanical characteristic occurs between these portions. This problem of irregularity of mechanical characteristic in a half-finished stabilizer is serious in a case of a stabilizer product using a hollow member.
In this situation, in order to decrease the temperature difference between the inner portion and the outer portion of the curved part of a half-finished stabilizer during current application heating, using the technology disclosed by Patent Document 1 may be considered. In the technology disclosed by Patent Document 1, a current application heating process is performed to perform burning coating, and it is presented that, by controlling the rate of temperature rise, setting the rate within a range 10-30° C./sec, before the burning coating in the current application heating process, the temperature difference between the inner portion and the outer portion of the curved part of a half-finished stabilizer can be decreased. In this case, the inner portion of the curved part of the half-finished stabilizer is locally cooled.